Evaporation-Induced Crystallization of Surfactants in Sessile Multicomponent Droplets

TL;DR

This study investigates how surfactant crystallization occurs in evaporating glycerol-water droplets containing SDS, revealing the process's universality and modeling it with diffusion and crystallization theories, which could impact understanding of surfactant behavior in evaporation systems.

Contribution

It introduces a combined experimental and modeling approach to understand surfactant crystallization during droplet evaporation, highlighting the role of preferential water evaporation and crystal formation.

Findings

Crystallization of SDS occurs due to water evaporation reducing solubility.

Crystals shield the droplet surface, stopping evaporation.

The diffusion and crystallization models agree with experimental data.

Abstract

Surfactants have been widely studied and used in controlling droplet evaporation. In this work, we observe and study the crystallization of sodium dodecyl sulfate (SDS) within an evaporating glycerol-water mixture droplet. The crystallization is induced by the preferential evaporation of water, which decreases the solubility of SDS in the mixture. As a consequence, the crystals shield the droplet surface and cease the evaporation. The universality of the evaporation characteristics for a range of droplet sizes is revealed by applying a diffusion model, extended by Raoult's law. To describe the nucleation and growth of the crystals, we employ the 2-dimensional crystallization model of Weinberg [J. Non-Cryst. Solids 1991, 134, 116]. The results of this model compare favorably to our experimental results. Our findings may inspire the community to reconsider the role of high concentration of surfactants in a multicomponent evaporation system.